(1) Field of the Invention
The present invention relates to a variable optical attenuator, and more particularly, to a variable optical attenuator capable of changing output optical power by means of varying the magnitude of optical coupling existing between input and output optical fibers through control of the polarizing state of light.
(2) Description of the Related Art
In association with an increase in the traffic over the Internet, the need to increase the capacity of optical communication has recently become urgent. One of the measures for increasing the capacity of optical communication is to increase a bit rate, and another measure is to employ wavelength division multiplexing (WDM). Prompt realization of an optical device which constitutes such a system is desired.
Here, WDM transmission is a technique for transmitting a plurality of wavelengths over a single optical transmission line (e.g., an optical fiber), wherein data are transferred at respective wavelengths, to thereby increase the capacity of communication. However, when data are transmitted through the optical fiber, propagation loss differs from one wavelength to another, and after transmission over a long distance changes arise in optical levels of the respective wavelengths.
When a branch device or an erbium-doped fiber (EDF) amplifier is used in the optical transmission line, this phenomenon becomes more noticeable. For this reason, optical levels at respective wavelengths must be made constant before optical transmission is performed. A solution for this is a technique (called “pre-emphasis”) for controlling an optical output achieved at the time of transmission beforehand such that an optical level achieved after transmission becomes constant, through use of a variable optical attenuator (hereinafter also called an “optical attenuator”), or the like, which controls levels of individual wavelengths. However, under the assumption that WDM transmission would be performed, optical levels must be set for respective wavelengths (channels). Hence, there must be provided an optical attenuator capable of varying optical power on a per-channel basis.
However, under present circumstance, there are many cases where optical attenuators are provided on a per-channel basis, thereby rendering devices, such as optical repeaters, bulky and incurring a cost hike. A technique described in Patent Publication 1 has hitherto been proposed as a measure for making the device compact. Specifically, as shown in FIGS. 16A and 16B, development has been pursued to constitute, as a single device, an optical attenuator capable of varying individual optical power levels of a plurality of channels through use of an optical waveguide device of planar type (or a planar lightwave circuit: PLC) 100. FIG. 16A is a top view of the optical attenuator, and FIG. 16B is a side view of the optical attenuator.
In the optical attenuator shown in FIGS. 16A and 16B, tape fibers (each being formed into a tape by stranding a plurality of optical fibers) 200 are connected to mutually-opposing input and output sections of the PLC 100 within a package (housing) 400. A desired voltage is applied, by way of electrical terminals 300, to electrodes provided in equal number to channels within the PCL 100, thereby changing the refractive index of a waveguide on a per-channel basis in order to change optical power.
Patent Publication 2 describes a conventional “handwritten input display device” which enables handwritten input and display of an image and a character by means of utilizing a phenomenon of changing a polarizing state of light through control of arrangement of liquid-crystal molecules.                [Patent Publication 1]JP-A-2000-180803        [Patent Publication 2]JP-A-63-201815        
However, the above-described planar lightwave device 100 usually requires micromachining of a quartz substrate through reactive ion etching (RIE) or like processing, thus incurring costs. Further, sufficient miniaturization of the lightwave device cannot be said to have been achieved, for reasons of a limitation on the micromachining technique.